1. Field of the Invention
The present invention relates to an optical modulation system including an optical modulator, and more particularly to an optical modulation system capable of keeping a highly stable bias voltage applied to an optical modulator for performing a light intensity modulation.
All of patents, patent applications, patent publications, scientific articles and the like, which will hereinafter be cited or identified in the present application, will, hereby, be incorporated by references in their entirety in order to describe more fully the state of the art, to which the present invention pertains.
2. Description of the Related Art
For realizing an optical communication using an optical fiber, a modulation in intensity of light based on data signals has generally been made. This light intensity modulation is to vary the light intensity, for example, light-on and light-off, over times. In order to obtain a higher transmission rate of not less than 10 GHz in bit rate, an external modulator has been used. Typical examples of the external modulator may be a Mach-Zehnder optical modulator or an electric field absorption optical modulator (EA-modulator), which exhibits a small chirping in the light intensity modulation. The chirping is a variation in frequency of the light due to another variation in intensity of the light. The small chirping is preferable for realizing the optical communication. For a long-distance optical communication, the Mach-Zehnder optical modulator is useful. Notwithstanding, it is disadvantageous that the Mach-Zehnder optical modulator allows a undesired variation in bias voltage from an intended or set voltage level due to any factors such as temperature variation, long-term field application and time-passing. The undesired variation in bias voltage causes variation in transmission characteristics over times.
Japanese laid-open patent publication No. 2000-162563 discloses a method for obtaining a desired stability in bias voltage, wherein a low frequency signal is superimposed over an electric driving signal to detect an amount and a direction of the variation of operating-point, for the purpose of feed-back control of the bias voltage based on the detected amount and direction.
FIG. 1 is a diagram illustrative of a structure of a conventional optical modulation system including a Mach-Zehnder optical modulator. Upon receipt of an input of an electrical signal 7 into a driver amplifier 55, the driver amplifier 55 output first and second electric driving signals 7A and 7B which are complementary to each other. The first electric driving signal 7A is supplied to a first signal electrode 51A of a Mach-Zehnder optical modulator 51. The second electric driving signal 7B is supplied to a second signal electrode 51B of the Mach-Zehnder optical modulator 51. A low frequency oscillator 4 supplies a low frequency signal to a low frequency superimposing circuit 54. A bias supply circuit 53 also supplies a bias voltage to the low frequency superimposing circuit 54. The low frequency superimposing circuit 54 superimposes the low frequency signal to the bias voltage, and supplies a superimposed bias voltage to the first signal electrode 51A of the Mach-Zehnder optical modulator 51. The superimposed bias voltage is a bias voltage superimposed with the low frequency signal.
A light source 1 emits a continuous wave light which is inputted into the Mach-Zehnder optical modulator 51. The light source 1 may typically comprise a photo-diode which emits a continuous wave light. The Mach-Zehnder optical modulator 51 performs a light intensity modulation of the inputted continuous wave light based on the first and second electric signals 7A and 7B as applied to the first and second signal electrodes 51A and 51B with the superimposed bias voltage. The Mach-Zehnder optical modulator 51 supplies an intensity-modulated light signal to an optical branch circuit 2. The optical branch circuit 2 divides the intensity-modulated light signal into a first intensity-modulated light signal 2A as a transmission signal and a second intensity-modulated light signal 2B as a feed-back signal.
The second intensity-modulated light signal 2B as a feed-back signal is supplied to a photoelectric converter 3 and converted into an electric feed-back signal by the photoelectric converter 3. The photoelectric converter 3 may typically comprise a photo-diode. The electric feed-back signal is supplied to an amplifier 5. The amplifier 5 supplies an amplified electric feed-back signal to a phase comparator 6. The low frequency oscillator 4 also supplies the low frequency signal to the phase comparator 6. The phase comparator 6 performs a synchronous detection by comparing the low frequency signal to the electric feed-back signal. The phase comparator 6 supplies a synchronously detected signal to a low pass filter 52. The low pass filter 52 extracts a direct current voltage error signal and supplies the direct current voltage error signal to the bias supply circuit 53. The bias supply circuit 53 generates a bias voltage based on the direct current voltage error signal and supplies the bias voltage to the low frequency superimposing circuit 54. The low frequency superimposing circuit 54 superimposes the low frequency signal to the bias voltage, and supplies a bias voltage superimposed with the low frequency signal to the first signal electrode 51A of the Mach-Zehnder optical modulator 51.
FIG. 2 is a view illustrative of a relationship of an extinction characteristic of the conventional optical modulation system with reference to electric driving signals amplitude-modulated by a low frequency signal superimposed to the bias voltage. The extinction characteristic is represented by a variation in light intensity over the applied voltage amplitude-modulated with the low frequency signal. Namely, the extinction characteristic means the dependency of the output light intensity upon the applied voltage level. As described above, the bias voltage superimposed with the low frequency signal is supplied to the first signal electrode 51A of the Mach-Zehnder optical modulator 51. The low frequency signal modulates the amplitudes of the complementary first and second electric driving signals 7A and 7B, which are applied to the first and second signal electrodes 51A and 51B of the Mach-Zehnder optical modulator 51 for driving the Mach-Zehnder optical modulator 51. The complementary first and second electric driving signals 7A and 7B are amplitude-modulated by the low frequency signal superimposed to the bias voltage, while the complementary first and second electric driving signals 7A and 7B have a full amplitude “2Vπ” which is defined to be a potential difference between adjacent two minimum points of the light intensity or between adjacent two maximum points of the light intensity. The low frequency signal has a constant frequency of “f0”, and a wavelength of “1/f0”.
FIG. 2 illustrates typical three different states of extinction characteristic, which are represented by a continuous line with mark (a), a dotted line with mark (b) and a broken line with mark (c). In a first extinction characteristic state represented by the continuous line with mark (a), the light intensity takes a minimum value or a minimum point at the direct current bias voltage free of superimposition of the low frequency signal. This means that the first extinction characteristic state represented by the continuous line with mark (a) is optimum for a duo-binary modulation. The control to the operating point is so made that the minimum point of the light intensity always corresponds to the direct current bias voltage free of superimposition of the low frequency signal. The above-described full amplitude “2Vπ” or full width “2Vπ” of the complementary first and second electric driving signals 7A and 7B is essential.
FIG. 3A is a diagram illustrative of an output light waveform represented by a variation in light intensity over times in the first extinction characteristic state represented by the continuous line with mark (a) in FIG. 2. FIG. 3B is a diagram illustrative of an output light waveform represented by a variation in light intensity over times in the second extinction characteristic state represented by the dotted line with mark (b) in FIG. 2. FIG. 3C is a diagram illustrative of an output light waveform represented by a variation in light intensity over times in the third extinction characteristic state represented by the broken line with mark (c) in FIG. 2.
In the first extinction characteristic state represented by the continuous line with mark (a) in FIG. 2, as shown in FIG. 3A, the output light waveform has a wavelength of “1/(2f0)” and a frequency of “2f0”, wherein “f0” is the frequency of the low frequency signal superimposed to the complementary first and second electric driving signals. The reason why the output light waveform is characterized by “2f0” or the double of the frequency “f0” of the low frequency signal is appearance of a reflecting effect of the low frequency signal at the maximum points of the light intensity as shown in FIG. 2. For this reason, the direct current voltage error signal obtained by the synchronous detection by the phase comparator 6 is thus zero “0”.
The first extinction characteristic state may be shifted to either the second extinction characteristic state represented by the dotted line with mark (b) in FIG. 2 or the third extinction characteristic state represented by the broken line with mark (c) in FIG. 2.
In the second extinction characteristic state represented by the dotted line with mark (b) in FIG. 2, as shown in FIG. 3B, the output light waveform has a wavelength of “1/(f0)” and a frequency of “f0”, wherein “f0” is the frequency of the low frequency signal superimposed to the complementary first and second electric driving signals. The reason why the output light waveform is characterized by the frequency “f0” of the low frequency signal is no appearance of any reflecting effect at the maximum points of the light intensity as shown in FIG. 2. For this reason, the direct current voltage error signal obtained by the synchronous detection by the phase comparator 6 is thus non-zero-value which depends upon a variation amount of the operating point.
In the third extinction characteristic state represented by the broken line with mark (c) in FIG. 2, as shown in FIG. 3C, the output light waveform has the wavelength of “1/(f0)” and the frequency of “f0”, wherein “f0” is the frequency of the low frequency signal superimposed to the complementary first and second electric driving signals. The output light waveform shown in FIG. 3C has an inverted phase to the output light waveform shown in FIG. 3B. Namely, the output light waveform shown in FIG. 3C is shifted in phase by “π” from the output light waveform shown in FIG. 3B. The reason why the output light waveform is characterized by the frequency “f0” of the low frequency signal is no appearance of any reflecting effect at the maximum points of the light intensity as shown in FIG. 2. For this reason, the direct current voltage error signal obtained by the synchronous detection by the phase comparator 6 is thus non-zero-value which depends upon a variation amount of the operating point. The direct current voltage error signal obtained in the third extinction characteristic state represented by the broken line with mark (c) in FIG. 2 has an opposite sign to the direct current voltage error signal obtained in the second extinction characteristic state represented by the dotted line with mark (b) in FIG. 2.
As described above, the output light waveform shown in FIG. 3C has an inverted phase to the output light waveform shown in FIG. 3B. Namely, the output light waveform shown in FIG. 3C is shifted in phase by “π” from the output light waveform shown in FIG. 3B. The direct current voltage error signal obtained in the third extinction characteristic state represented by the broken line with mark (c) in FIG. 2 has an opposite sign to the direct current voltage error signal obtained in the second extinction characteristic state represented by the dotted line with mark (b) in FIG. 2. These means that the sign of the direct current voltage error signal detected by the synchronous detection by the phase comparator 6 indicates the direction of the variation of the operating point or the direction of shifting the first extinction characteristic state to either the second or third extinction characteristic state. Namely, detection of the sign of the direct current voltage error signal detects the direction of the variation of the operating point. Also, the absolute value of the direct current voltage error signal detected by the synchronous detection by the phase comparator 6 indicates the magnitude or the amount of the variation of the operating point or the magnitude or the amount of shifting the first extinction characteristic state to either the second or third extinction characteristic state. Namely, detection of the absolute value of the direct current voltage error signal detects the magnitude or the amount of the variation of the operating point.
Accordingly, both the direction and the magnitude or amount of the variation of the operating point can be detected, so that the bias voltage level can be controlled based on the sign and the absolute value of the direct current voltage error signal detected by the synchronous detection by the phase comparator 6, whereby the operating point can be stabilized. Namely, the minimum value point of the extinction characteristic may be taken as the stable operating point.
Similarly, the maximum value point of the extinction characteristic may also be taken as the stable operating point. In this case of taking the maximum value point as the stable operating point, the sign, for example, plus and minus, of the direct current voltage error signal detected by the synchronous detection is opposite to the sign of the direct current voltage error signal in the above case of taking the minimum value point as the stable operating point.
The above-described conventional optical modulation system has a disadvantage in difficulty in controlling or suppressing an undesired variation or fluctuation of the operating point. This difficulty is caused by the following two issues.
The first issue is that the above-described full amplitude “2Vπ” or full width “2Vπ” of the complementary first and second electric driving signals 7A and 7B for driving the Mach-Zehnder optical modulator 51 is essential. The above-described full amplitude “2Vπ” or full width “2Vπ” makes a tolerance small, because the reflecting effect of the low frequency signal at the minimum value point or the maximum value point of the is utilized to detect the stable operating point.
The second issue is that a locking range of the operating point or a follow-range of the operating point is limited due to a finite vias voltage which can be supplied from the bias supplying circuit.
In the above circumstances, the development of a novel optical modulation system free from the above problems is desirable.